The present invention relates to a method of making elements of a magnetic circuit in a loudspeaker such as yoke and plate.
In an electrodynamic loudspeaker, which is widely used, an electric signal is converted into an acoustic signal by magnetic flux generated by a magnet and magnetic force caused by the electric signal.
Referring to FIG. 2 a cone loudspeaker, which is one type of electrodynamic loudspeaker, has a yoke 12 having an integral center pole 11 and a yoke base 13, an annular magnet 14 mounted on the yoke base 13, and an annular plate 15 mounted on the magnet 14, thereby forming a magnetic circuit. On the plate 15 is mounted a conical frame 20 which supports a conical diaphragm 23 around an upper edge thereof at an edge 21. A lower edge of the diaphragm 23 is disposed in a magnetic gap G formed between the center pole 11 of the yoke 12 and the plate 15 and secured to a voice coil 22 which is supported by a damper 24. A lead 30 connects the voice coil 22 to an external terminal 31 which is connected to an external source to feed audio current to the voice coil 22. When applied with the audio current, the voice coil 22 generates a magnetic force, which moves the voice coil 22. The diaphragm 23 is thus caused to vibrate to produce sound waves.
In the speaker, the base 13 has a uniform thickness as shown in FIG. 2. In such a speaker there is a large leakage flux, which causes the magnetic flux generated in the magnetic gap G to reduce. Japanese Utility Model Publication 46-8272 discloses a yoke where the thickness of which is reduced toward the peripheral portion thereof to prevent the reduction of the magnetic flux.
The inventors of the present invention have proposed a magnetic circuit in Japanese Patent Application 2-280773 which is shown in FIG. 3. A yoke 12a and plate 15a are slanted toward the periphery. The upper surface of the plate 15a has a slight curve 17 and the lower surface of a yoke base 13a has also a curve 16 so that each of the peripheral portion thereof is extremely thinned. A loudspeaker having such an arrangement is advantageous in that the magnetic flux flows mainly through the gap G so that a leakage flux .phi.1 between the plate 15a and the yoke base 13a, detouring the magnet 14, is decreased. A leakage flux .phi.2 which flows from the plate 15a to the center pole 11 is also decreased.
Such a yoke and a plate are manufactured by cold forging so as to form thin peripheral edges.
Referring to FIG. 4a, a roughly shaped workpiece W is disposed between an upper die 40 and a lower die 42, which together form an inner space for a desired shape of the yoke base 13a. The upper die 40 has a recess 41 having a curve which corresponds to the curve 16 of the yoke base 13a, and the lower die 42 has a recess 43 which corresponds to the contour of the center pole 11. When the upper die 40 is pressed against the lower die 42 at a pressure P, the workpiece W is shaped in accordance-with the inner shape of the dies 40 and 42 by plastic flow. Hence the yoke 12a having the appropriate curve 16 along the lower surface of the base 13a is obtained. The plate 15a having the curve 17 is manufactured in the same manner.
During the process, a part of the workpiece W extends in a small gap g formed between the upper and lower dies 40 and 42 to form a thin peripheral portion of the yoke base 13a. However, the quantity of the material entering the gap g is liable to differ in different peripheral places. As a result, as shown by dotted lines in FIG. 4b, the periphery of the yoke 12a has not a round shape so that the yokes having various dimensions are produced, which causes reduction of dimensional accuracy. The accuracy is further decreased as the peripheral portion becomes thinner.
If a smaller diameter d1 than a desired diameter d2 shown in FIG. 4b is used, the magnet 14 largely extends from the periphery of the yoke base 13a. On the other hand, if the yoke base 13a has a large diameter d3, the magnet 14 is positioned far inside the periphery of the yoke 12a. These inaccuracies cause the increase of leakage flux, reducing the efficiency of the effective magnetic flux generated in the gap G.